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Depending on the manufacturer of the furnae, it does make a big difference. Most manu. are looking for the resistance of the intake air pipe to match the flue side. If you look at carrier for ex. they state take the longer length of the 2 to determine flue size.

Also the biggest reason is for termination. in a two pipe system it is stated that you can be 1 foot away from a window or door. A one pipe system it HAS to be 4 feet away from window or door. this is to prevent funace pulling a vacuum on a mech room with insufficient combustion air. If a door or window were to be opened in the mech room it would then pull in flue gasses.

Originally posted by flyboy917 Just realized the PVC pipe that I "thought" was combustion air intake for my new 90% furnace is actually the flu.
The combustion air is actually coming from a PVC 90 on the side of the furnace. Is that cool?

Flyboy

Your unit was installed as per one of the methods allowed by Rheem, so it is proper.

Is is "cool"? No. For all of the reasons mentioned, it is not "cool". You are using contaminated, conditioned air to burn which causes a negative pressure in your home which causes unconditioned air (draft) to have to be drawn back into the home. Contaminated air in the home also causes issues with the furnace burners, ignitors, flame sensors and heat exchangers. So, no, it is not "cool".

As long as no combustion air piping is used there is no need for the vent and combustion air piping to be the same length. The most important factor when having the two pipe system is that the termination of the vent and the combustion air be within 12-14" of each other with the vent slightly higher then the combustion air. A better method is to use a concentric termination which puts the vent and combustion air termination in the proper pressure zone in relationship to each other.

If its in a big enough space with plenty of combustion air then one pipe is fine.
But I have that same scenario and the sensor on my 7 year old Armstrong furnace needs replacing every 2-3 years cause it pulls in contaminated air. Who knows what it is doing to the heat exchanger. There are surface rust spots at the heat exchanger near the in shot burners.
The other problem is that if I used 2 pipes the extremely cold air when it is below zero out would cause condensation and water at the burner area.
So I guess it depends on where you live and how cold it gets.
If you're installing in a extremely cold climate with sufficient combustion air then I would rather replace sensors every 2-3 years than replace burners and explain to the customer why all the water on the furnace.
If its moderate climate then I would go with 2 pipe every time.

The concentric termination kits greatly help prevent that condensation issue by warming up the combustion air.

Those contaminants are doing a lot more harm then tearing up the pressure switches. Contaminants in combustion air are real killers of condensing furnaces.

The fact that there are so many things that are harmful contaminants is the real issue. Moisture is a harmful contaminant when it hits superheated metal surfaces. Anything that has any odor to it that is not from nature is a chemical contaminant that turns into really nasty oxides when exposed to the extreme heat of HSIs, burners, flame sensors and heat exchangers.

The colder it is outside, the less moisture is in a cubic inch of air making it more desirable to burn. There are also no common household chemicals hanging around outside to destroy the furnace. Major contributors of hazardous chemicals in the home are;
* any arosol
* scented candles
* laundry detergent
* fabric softeners (the worst)
* plug in smelly thingies (2nd worst)
* kitty litter boxes
* humidifiers
* deoderants
* perfumes, after shave lotions and cologne
* any product that has a chemically produced scent

The efficiency rating is not based on how tight the house is or if a window is open. Similarly it costs more to run if you need to work harder to overcome drawing all the uncondioned air in and throwing good conditioned air out as has been explained.